1. Field of the Invention
The present invention relates to a hydraulic pressure control apparatus and more particularly to a hydraulic pressure control equipment such as a steering device, a suspension or the like installed on a car and in particular, a hydraulic pressure control apparatus, comprising a flow rate-variable valve for preventing the leakage of hydraulic fluid, preferably used as a braking liquid control apparatus such as an antilock braking fluid pressure control apparatus, a traction control braking fluid pressure control apparatus or the like.
2. Description of the Related Arts
Various flow rate-variable valves suitable for a hydraulic pressure control apparatus have been proposed. Description is made on the flow rate-variable valves proposed by the present applicant and disclosed in the Japanese Patent Laid-Open Publications Nos. 3-234987 and 3-223578.
Referring to FIG. 10, the flow rate-variable valve disclosed in the Japanese Patent Laid-Open Publication No. 3-234987 is described below. A sleeve 1 accommodates a spool 2 which is slidable in the sleeve 1. A duct 2B including an orifice 2A is disposed in the spool 2 to be urged by a spring 3 and an electromagnet (not shown). A passage 2C communicating with the duct 2B is formed on the surface of the spool 2.
The flow rate of the orifice 2A is determined by the difference in the pressure between one side of the orifice 2A communicating with an inlet port 1A and the other side thereof communicating with an outlet port 1B. The pressure difference depends on the urging force applied to the spool 2 by the spring 3 and the electromagnet. The spool 2 slides in the sleeve 1, thus communicating the inlet port 1A with the passage 2C or cutting the communication therebetween repeatedly to form a variable orifice. The variable orifice maintains the difference in the pressure between both sides of the orifice 2A. Therefore, in the flow rate-variable valve, the intensity of electric current is adjustably supplied to the electromagnet to change the urging force applied to the spool 2 by the spring 3 and the electromagnet. In this manner, the flow rate of the hydraulic fluid flowing from the inlet port 1A to the outlet port 1B can be successively controlled.
In the flow rate-variable valve disclosed in the Japanese Patent Laid-Open Publication No. 3-223578, as shown in FIG. 11, a groove 2D is formed on the peripheral surface of the spool 2 to be urged by an electromagnet (not shown), and a shoulder 2E of the spool 2 engages an annular sealing member 5 into which the groove 2D has been inserted. In this manner, a variable orifice is formed.
In the flow rate-variable valve of FIG. 11, the shoulder 2E is brought into close contact with the annular sealing member 5 or move away therefrom, thus communicating the inlet port 1A with a passage 2B of the spool 2 or cutting the communication therebetween repeatedly to form a variable orifice. The variable orifice maintains the difference in the pressure between both sides of the orifice 2A. Therefore, in the flow rate-variable valve, the intensity of electric current is adjustably supplied to the electromagnet to change the urging force applied by the electromagnet. In this manner, the flow rate of the hydraulic fluid flowing from the inlet port 1A to the outlet port 1B can be successively controlled.
An electromagnetic change-over valve is only fully opened or fully closed whereas in the flow rate-variable valve shown in FIGS. 10 and 11, the flow rate of hydraulic fluid can be continuously controlled by adjusting the intensity of electric current to be supplied to the electromagnet. Therefore, the flow rate-variable valve is capable of performing a preferable control over the flow rate of the hydraulic fluid in the hydraulic pressure control apparatus and in addition, generates a more quite sound than the electromagnetic change-over valve during operation.
In the flow rate-variable valve shown in FIG. 10, however, a sliding surface A is interposed between the inlet port 1A and the outlet port 1B. Therefore, a small amount of hydraulic fluid leaks from the inlet port 1A to the outlet port 1B via the sliding surface A.
In the flow rate-variable valve shown in FIG. 11, the variable orifice is formed by the engagement between the annular sealing member 5 and the shoulder 2E of the spool 2. Although the amount of hydraulic fluid which leaks from the inlet port 1A to the outlet port 1B in this flow rate-variable valve is smaller than that in the flow rate-variable valve shown in FIG. 10, the flow rate-variable valve is incapable of preventing the leakage of the hydraulic fluid.
An opened type flow rate-variable valve is used in addition to the closed type as shown in FIGS. 10 and 11. Even if it is excellent for the opened type flow rate-variable valve is superior to the closed type flow rate-variable valve in controlling the valve-opening/closing performance, it is necessary to keep supplying electric current thereto when the flow rate-variable valve is used as a discharge valve of an antilock braking fluid pressure control apparatus. Therefore, the opened type flow rate-variable valve is unsuitable for this kind of discharge valve. Similarly, it is necessary to keep supplying electric current to the opened type flow rate-variable valve when the flow-rate-variable valve is used as an introducing valve of a press-back type antilock braking fluid pressure control apparatus so as to introduce hydraulic fluid stored in an accumulator having a high pressure. It is also necessary to keep supplying electric current to the opened type flow rate-variable valve when the flow rate-variable valve is used as a hydraulic fluid-introducing valve in a brake traction fluid pressure control apparatus or in a leveling adjusting apparatus. Therefore, the opened type flow rate-variable valve is unsuitable for this kind of discharge valve.
The present applicant proposed a braking fluid pressure control apparatus comprising a plurality of discharge valves to accomplish a reliable operation thereof (see Japanese Patent Laid-Open Publication No. 63-61671).
In this braking fluid pressure control apparatus, a discharge valve comprising a plurality of closed type flow rate-variable valves may reduce the leakage amount of hydraulic fluid. In this case, too, the leakage of a small amount of the hydraulic fluid is unavoidable. A plurality of opened flow rate-variable valves are ineffective for preventing the leakage of the hydraulic fluid.